This invention relates generally to the art of automatic regulation of dc power supplies and, more specifically, to automatic switching regulated power supplies.
In the art of automatic regulation of dc power to a load, it has been the practice to use either a continuous analog control method or a switching control method, referred to as a "chopper." The continuous analog control method is referred to as a dissipative type controller in that the difference between the source power and the instantaneous load power requirement must be dissipated in the regulating device, such as a series transistor regulator. Therefore, in order to minimize power dissipation in the regulating device, especially for rather high power requirements, it has been the practice to use a switching regulator in which the average power supplied from a dc source to a dc load is regulated by a series switch that repetitively opens and closes thereby "chopping" the current which flows between the two. Control is achieved by varying the relative on time or duty cycle. In automatic regulation the duty cycle is determined by a circuit which compares the load voltage to a reference voltage.
When using a transistor as the switching element in a switching regulator, the transistor is operated at or near saturation, where its dissipation is minimized. Thus, in high power applications, requiring a number of regulating elements in a parallel array to handle the high currents, it is desirable to use the switching or chopper method in order to reduce the number of switching elements required due to the reduced switching element dissipation. However, when a switching regulator is used to regulate dc power to an inductive load, some means must be provided for absorbing the energy stored in the inductance when the circuit is interrupted, otherwise the inductive voltage spikes may destroy the switching elements. This may be accomplished by connecting a freewheeling diode in parallel with the load so that the stored energy is dissipated in the load itself.
A regulator of this type may be used successfully to regulate the current to an inductive load as long as the current is to be at a fairly constant level. However, in many applications it is desirable to regulate large fluctuations in load current in an inductive load very quickly wherein the load dissipation rate is very low. One example is the vertical field winding of a tokamak fusion reactor which is used to position the plasma within the plasma chamber. These windings are highly inductive with very little resistance for power dissipation. Another example is an electric motor driven vehicle where it is not only desirable to regulate fast changes in current applied to the motor but to return energy to the power supply during rundown or regenerative braking while minimizing power dissipation in the regulating device and providing smooth current regulation during all phases of operation of the vehicle. The conventional switching regulated power supplies do not provide this type of control. Thus, there is a need for an automatic switching regulator for inductive loads which takes advantage of the desirable low dissipation in the control element while allowing fast current changes, both runup and rundown, and the added advantage of dumping energy from the load back into the source during current rundown.